In recent years, the installation of surveillance cameras in shops, shopping quarters, housing complexes, and the like as well as the installation of drive recorders in business vehicles, and the like have become wide spread, and the number of incidents where video information is used as evidence has increased. Furthermore, the recording of conversations between customers and operators and the retaining of the recorded conversations as evidence to resolve trouble occurring with agreements made over telephone and support services provided by telephone has become common practice.
At present, if a shop is to use video information as evidence, the shop submits a videotape or images without being processed. Advancements in the digital storage of images facilitate tampering and/or editing of images. If the video information is treated as evidence, the shop is requested to add third-party certification such as a signature or a timestamp.
Moreover, consequent to the large data volume, compression techniques may be used when images or motion pictures are submitted. For example, Joint Photographic Experts Group (JPEG) is an image compression format, and Motion JPEG where each video frame is in the JPEG format is a motion-picture compression format. In the JPEG format, each image is divided into multiple areas, and pixel values of each area are retained as differential values from those of an area just before the former area to make the statistical bias of each pixel value large, thereby improving the compression efficiency.
For example, as a technique for detecting third-party tampering, a technique is known for generating digest information for each video frame, and an electronic signature is added to the digest information. The digest information corresponds to a hash that is referred to as a “message digest” and that is calculated using a cryptographically secure one-way hash function. A technique is also known for dividing each original motion picture into a group of pictures (GOPs) that are reproducible minimum units, and for generating hashes for the obtained GOPs, thereby enabling a third party to certify that extracted motion pictures are part of the original motion pictures and have not be tampered with.
Moreover, there has been demand to make data public while sanitizing contents that are desired to be kept private from the perspective of protecting privacy. In relation to this, a technique is known for dividing each image into multiple areas, sanitizing contents of the areas, and generating digest information for each of the areas, thereby enabling the images to be made public in a state where the contents of some areas has been sanitized (see, for example, Published Japanese-Translation of PCT Application, and Publication No. 2010/97923 Japanese Laid-Open Patent Publication Nos. 2009-152713 and 2006-180472).
However, the above conventional techniques have the following problems. When a verification device verifies the originality of the images obtained from the original data according to the image compression format and made public with contents of part of the areas sanitized, it disadvantageously takes a long time for a verification process because the verification device verifies each sanitized area as a signature target of the electronic signature.